Processes for the production of composite plastic parts which have cellular cores and solid cover layers are already known.
German Auslegeschrift No. 2,127,582 describes a process which results in cellular composite parts having compact, defect-free surfaces, by successively introducing several starting substances into a closed mold with a rigid cavity. In this process, at least one foamable and either a second foamable or a nonfoamable plastic charge is used, with the second charge compressing the first charge and forming the surface of the composite part. The timing of successive charges is selected in order that the second charge does not penetrate the first charge, but rather compresses it (depending on its mass and volume), and forms a good chemophysical bond with the first, because the first charge is not yet fully reacted and has still a sufficient tackiness and is still compressible. Consequently, the introduction of a plastics material containing a blowing agent into a plastic material which does not contain a blowing agent is no longer necessary for the production of a composite plastics article and neither is the expandable mold cavity (see German Auslegeschrift No. 2,442,227).
German Auslegeschrift No. 2,127,582 also particularly describes, in three embodiments, combinations of polyurethanes of a varying structure and hardness, as well as combinations of polyurethanes with other polymerization and polycondensation plastics. These embodiments, however, are restricted to a description of the procedure for filling the cavity of the mold, during which operation, the number and relative position of mixing heads used may vary. There are no details, in the very detailed description of the process of the reference, concerning the release from the mold of the composite plastics part. It is merely stated that the composite part, consisting of the first and second charges, has the shape and dimensions of the mold cavity, and that the composite part is removed from the mold after shaping.
However, it has now been found that during the production of composite parts from combinations of cellular polyurethane plastics, according to the teaching of German Auslegeschrift No. 2,127,582, it is only possible to release the composite part from the mold by damaging the part, as the surface of the part bonds to the surface of the cavity. Thus, composite parts, consisting of a first charge of a foamable polyurethane foam, which ought to simplify the release from the mold due to its hardness, and a second charge of a foamable or non-foamable polyurethane material, could not be removed from the mold without damaging the composite part.
Regardless of whether the second charge was a flexible, impact-resistant, semi-rigid, rigid, or very rigid, polyurethane plastic, a perfect release from the mold was impossible. Even in the case of a combination of a rigid polyurethane foam with a very rigid polyurethane as the second and outer layer (the combination which would be expected to be the most suitable), a smooth release could not be achieved.
The object of the present invention is to provide a process for the production of a composite part of cellular plastic containing urethane and/or isocyanurate groups, which does not have mold-release problems.